1. Field of the Invention
The present invention relates to a magnetic recording medium, and more specifically to a high recording density magnetic recording medium with excellent surface smoothness, electromagnetic conversion characteristics and durability
2. Disclosure of the Related Art
In recent years, the magnetic recording media for digital recording to release from signal deterioration arising from repeated copying have become common. Furthermore, in order to cope with increases in recording data volumes, magnetic recording media with increased recording densities are required. In order to increase the recording density, factors such as media thickness loss and self-demagnetizing loss must be considered, and for these reasons, it is desired to reduce in the film thickness of the magnetic layer.
When the thickness of the magnetic layer is reduced, the surface roughness of the support is reflected on the surface of the magnetic layer, causing a loss of smoothness in the magnetic layer surface, and a deterioration of the electromagnetic conversion characteristics. Consequently, a non-magnetic layer is provided on the surface of the support as an undercoat layer, and the magnetic layer is then provided on the surface of this non-magnetic layer. As a result, the surface of this non-magnetic layer provided beneath the magnetic layer need to be formed with smooth, to obtain a magnetic layer with a smooth surface.
Furthermore, when the thickness of the magnetic layer is reduced, the quantity of lubricant that can be held within the magnetic layer decreases markedly. The addition of large quantities of lubricants to the magnetic layer undesirably results in such a problem as blooming, and a satisfactory level of durability cannot be achieved solely through the addition of the maximum quantity of lubricant which can be added to the magnetic layer.
Japanese Patent Publication No. Hei 1-25136 discloses the use of combinations of fatty acids, fatty acid esters and fatty acid amides as the lubricant for a magnetic layer on a support. However, even if a lubricant combination such as that disclosed in the above publication is applied to an extremely thin magnetic layer of 0.2 μm or less, the absolute quantity of lubricants that can be incorporated within the layer is insufficient due to remarkable thinness of the magnetic layer, and adequate levels of durability and reliability cannot be obtained. Furthermore, if the entire quantity of lubricants deemed necessary is added to the thin magnetic layer, then problems such as lubricant blooming under conditions of high temperature and high humidity become far more likely.
As a result, lubricants are added to, and held within the non-magnetic layer to become a lower layer of the magnetic layer. The lubricants within the non-magnetic layer are supplied gradually to the surface of the medium, depending on the environment in which the magnetic recording medium is used, and consequently the durability improves.
An example of a magnetic recording medium in which lubricants are added to the lower non-magnetic layer is detailed in Japanese Patent Laid-Open Publication No. Hei 10-334450 which discloses the lower non-magnetic layer contains acicular particles of hydrated iron oxide such as acicular goethite dispersed in a binder, wherein the acicular particles of hydrated iron oxide has the average long axial length of 0.2 μm or less and the average short axial length of 0.04 μm or less, and the lubricants for the lower non-magnetic layer utilize fatty acids and fatty acid esters.